Nitrogen-fixing bacteria convert atmospheric nitrogen gas, which is almost inert, into solid nitrogen compounds that can then be absorbed by plants. These bacteria help fertilize the plants.
Nitrogen cannot be used by most organisms unless it is combined with or “fixed” with other elements to form nitrogenous compounds. Nitrogen fixation occurs in nature either due to lightning or nitrogen-fixing bacteria.
There are two broad groups of nitrogen-fixing bacteria. The first group includes the free-living bacteria that are not attached to any plant in particular and are found in the soil or in water. Examples of this group include the genera Clostridium and Azotobacter, as well as some blue-green algae. The second group includes bacteria that live on the roots of certain legumes. This consists of bacteria from the Rhizobium genus, which live in nodules found on the root of the plants.
The relationship between the plant and the bacteria is a symbiotic one. The plant supplies the bacteria with food. The bacteria helps fertilize the soil in return by forming ammonium compounds through the fixation of atmospheric nitrogen. The compounds are secreted into the soil, where they can be absorbed by the legumes, as well as other plants growing in the same area.